Signal processor and radiation detection device

ABSTRACT

According to an embodiment, a signal processor includes an integrator, a differentiator, a zero cross detector, a pile-up detector, an event interval detector, a counter, and a creator. The integrator is configured to calculate charge of current from a photoelectric converter for an incident radiation. The differentiator is configured to calculate a differential value of the current. The zero cross detector is configured to detect a zero cross of the differential value. The pile-up detector is configured to detect pile-up of the current based on the zero cross. The event interval detector is configured to detect, based on the zero cross and pile-up, an event interval of the radiation entering. The counter is configured to count, based on the charge and pile-up, the respective numbers of events according to the charge and the event interval. The creator is configured to create histograms for the numbers of events.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT international Application Ser.No. PCT/JP2014/084414, filed on Dec. 25, 2014, which designates theUnited States; the entire contents of which are incorporated herein byreference.

FIELD

An embodiment of the present invention relates to a signal processor anda radiation detection device.

BACKGROUND

Recently, a silicon-based photomultiplier is actively developed, andalso a weak light detection system using a scintillator and aphotomultiplier (such as a radiation detection device adapted to detectan X-ray and the like) is further along in development. For example, theradiation detection device is utilized in computed tomography (CT) andthe like and enables tomography of a patient, a luggage, or the like.Especially, as a photoelectric conversion element to be a detection unitof the photomultiplier, a SiPM formed by connecting, in series, anavalanche photodiode (APD) and a quench resistor has a high S/N ratioand a high dynamic range and achieves low-voltage drive. Such aradiation detection device utilizing the photoelectric conversionelement detects current from the photoelectric conversion element, andacquires charge and voltage by integrating the current, and then thevoltage (charge) is sampled and held to be subjected to AD conversion.An obtained digital signal is utilized to create a histogram and thelike by signal processing.

On the other hand, in a radiation detection device of a photon countingsystem, an arrival rate of an X-ray entering an scintillator isestimated to be about 10⁸ [cps], and a circuit that can measure datawith high-speed and high-energy resolution by several hundreds ofchannels is demanded. Furthermore, a counting rate that can be detectedby the above-described radiation detection device is varied by arecovery time of the photoelectric conversion element, conversioncapacity of an AD converter, or the like. However, in order to shortenthe recovery time of the photoelectric conversion element, there may bean exemplary method of reducing a time constant by setting a small valuefor a quench resistor of the photoelectric conversion. However, in thecase where the value of the quench resistor is too small, quenchingoperation may not be able to be performed. Therefore, shortening therecovery time has a limit. Due to this, a so-called pile-up may occurwhen a phenomenon (hereinafter referred to as event”) in which a photonof radiation (or scintillation light converted by scintillator) entersthe photoelectric conversion element occurs within the recovery time ofthe photoelectric conversion element.

Furthermore, a probability of event occurrence occurring in theradiation detection device is based on Poisson distribution. Therefore,in the case where an event interval is, for example, an interval shorterthan an average arrival time 10 [ns] (equivalent to 10⁸ [cps] describedabove) of the radiation, the probability of event occurrence is aboutsix out of ten, and even in the case of 4 [ns] or less, the probabilityof event occurrence is about three out of ten. Therefore, the ADconverter having high-speed AD conversion capacity is necessary in orderto detect an event, but the high-speed AD converter consumes a largeamount of electricity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary structure of a radiationexamination device;

FIGS. 2A and 2B are diagrams illustrating an exemplary structure of aradiation detector;

FIG. 3 is a diagram illustrating a circuit configuration of a signalprocessor;

FIG. 4 is an explanatory diagram for Poisson distribution related to anarrival probability of radiation;

FIG. 5 is an explanatory diagram for a case where an interval between anormal waveform and a pile-up waveform is wide;

FIG. 6 is an explanatory diagram for a case where the interval betweenthe normal waveform and the pile-up waveform is narrow;

FIG. 7 is a timing chart from entrance of radiation to application of ADconversion;

FIG. 8 is a diagram illustrating an exemplary histogram for charge andthe number of events; and

FIG. 9 is a diagram illustrating an exemplary histogram for an eventinterval and the number of events.

DETAILED DESCRIPTION

According to an embodiment, a signal processor includes an integrator, adifferentiator, a zero cross detector, a pile-up detector, an eventinterval detector, a counter, and a creator. The integrator isconfigured to calculate charge by integrating current from aphotoelectric converter that converts to the current based on anincident radiation. The differentiator is configured to calculate adifferential value by differentiating the current. The zero crossdetector is configured to detect a zero cross of the differential value.The pile-up detector is configured to detect pile-up relative to thecurrent based on the zero cross. The event interval detector isconfigured to detect, based on the zero cross and the pile-up, an eventinterval that is a time period between events in which the radiationenters the photoelectric converter. The counter is configured to count,based on the charge and the pile-up, number of events according to thecharge, and number of events according to the event interval. Thecreator is configured to create a histogram for the number of eventsrelative to the charge, and a histogram of the number of events relativeto the event interval.

In the following, a signal processor and a radiation detection deviceaccording to an embodiment of the present invention will be described indetail with reference to the drawings. Furthermore, note that a sameportion is denoted by a same reference sign in the following drawings.However, since the drawings are schematically illustrated, a specificconfiguration should be determined in consideration of the followingdescription.

FIG. 1 is a diagram illustrating an exemplary structure of a radiationexamination device. An entire structure of the radiation examinationdevice 1 will be described with reference to FIG. 1.

As illustrated in FIG. 1, the radiation examination device 1 includes: aradiation tube 11; and a radiation detection device 10 provided in amanner facing the radiation tube 11.

The radiation tube 11 is a device to irradiate the facing radiationdetection device 10 with a radiation beam 11 a such as an X-ray in afan-like form. The radiation beam 11 a emitted from the radiation tube11 transmits a test object 12 on a gantry not illustrated, and entersthe radiation detection device 10.

The radiation detection device 10 is irradiated by the radiation tube11, and at least a part thereof receives the radiation beam 11 a that atleast partly transmits the test object 12 on an incident surface 20 a,and converts the radiation beam to scintillation light including atleast part of ultraviolet, visible light, and infrared of the radiation,and then detects the scintillation light as an electrical signal. Theradiation detection device 10 includes: a plurality of radiationdetecting units 20 arrayed in a substantially arc shape; a collimator 21mounted on the incident surface 20 a side of the radiation detectingunit 20, and a signal processor 22 connected to an electrode on anopposite side of the radiation tube 11 side of each of the radiationdetecting units 20 via a signal line 23.

The radiation detecting unit 20 converts the radiation (radiation beam11 a) entering the incident surface 20 a to scintillation light, and thescintillation light is converted to an electrical signal (current) by alater-described photoelectric conversion element 32 (photoelectricconversion).

The collimator 21 is an optical system mounted on the incident surface20 a side of the radiation detecting unit 20 and adapted to refract theradiation such that the radiation parallelly enters the radiationdetecting unit 20.

The signal processor 22 receives the electrical signal (current)photoelectrically converted by each of the radiation detecting units 20via the signal line 23, thereby detecting an event and calculatingenergy of the radiation entering each of the radiation detecting units20 from a received current value.

Then, the radiation tube 11 and the radiation detection device 10 arearranged so as to be rotated around the above-described test object 12.Thus, the radiation examination device 1 can generate a cross-sectionalimage of the test object 12. Meanwhile, the radiation examination device1 including the radiation detection device 10 can be applied to not onlytomogram images of human bodies, animals, and plants but also variouskinds of examination devices such as a security device which performsfluoroscopy for inside of an article, and the like.

FIGS. 2A and 2B are diagrams illustrating an exemplary structure of aradiation detector. Structures of the radiation detecting unit 20 andradiation detector 30 will be described with reference to FIGS. 2A and2B. FIG. 2A is a structural diagram of a plurality of radiationdetecting units 20 arrayed in the substantially arc shape, and FIG. 2Bis a schematic structural diagram of the radiation detector 30 includedin the radiation detecting unit 20.

As illustrated in FIG. 2A, the plurality of radiation detecting units 20is formed by being arrayed in the substantially arch shape, and thecollimator 21 is arranged on the incident surface side of the radiation.As illustrated in FIG. 2B, the radiation detecting unit 20 has theradiation detector 30 fixed onto an element supporting plate 24. Theradiation detector 30 includes a photoelectric conversion layer 31provided with a plurality of photoelectric conversion elements 32 insidethereof and a scintillator 33 adapted to convert radiation toscintillation light. The photoelectric conversion layer 31 and thescintillator 33 have a stacking structure in which an incident surfaceside of the photoelectric conversion layer 31 is bonded to a lightemission surface side of the scintillator 33 by a bonding layer.

The scintillator 33 includes light reflection plates 34 formed at apredetermined pitch in two directions intersecting with each other. Thephotoelectric conversion layer 31 and the scintillator 33 are definedinto a plurality of photoelectric converters 35 arrayed in a matrix bythe light reflection plates 34. A plurality of photoelectric conversionelements 32 is respectively included in the plurality of photoelectricconverters 35, and detection of an event, detection of energy, and thelike of the incident radiation are performed per the photoelectricconverter 35.

The radiation such as the X-ray emitted from the radiation tube 11(refer to FIG. 1) enters the scintillator 33 of the radiation detector30. The radiation is converted by the scintillator 33 to thescintillation light including at least any one of ultraviolet, visiblelight, and infrared having a wavelength longer than the radiation as anelectromagnetic wave. The converted scintillation light passes thescintillator 33 while being reflected by the light reflection plate 34,and is directed to the photoelectric conversion layer 31.

The scintillation light emitted from the scintillator 33 enters theplurality of photoelectric conversion elements 32 formed on thephotoelectric conversion layer 31. The photoelectric conversion element32 forms electrical connection in a direction from a cathode side to ananode side of the APD of the photoelectric conversion element 32(reverse bias direction) due to avalanche breakdown caused by entranceof the scintillation light (photon).

The photoelectric conversion element 32 is applied with voltage to bethe reverse bias by the signal processor 22 (refer to FIG. 1). At thispoint, since electrical connection in the reverse bias direction is madeby entrance of the photon into the photoelectric conversion element 32as described above, current flows in the photoelectric conversionelement 32 (APD and a quench resistor connected in series thereto) inthe reverse bias direction. Then, the current is detected by the signalprocessor 22 via the signal line 23.

Here, a value of the current flowing in the photoelectric conversionelement 32 in the reverse bias direction is substantially not influencedby the number of incident photons (intensity of scintillation light).For example, in the case where one hundred of photons enter onephotoelectric conversion element 32, ten out of one hundred photonsenter each of ten photoelectric conversion elements 32 relative to thecurrent value flowing in the one photoelectric conversion element 32,and a total value of the current flowing in the ten photoelectricconversion elements 32 becomes ten times. Therefore, in order toaccurately detect intensity (current) of the radiation having enteredthe photoelectric converter 35 including the plurality of photoelectricconversion elements 32, the scintillation light is needed to uniformlyenter the plurality of photoelectric conversion elements 32 inside thephotoelectric converter 35. Thus, since the scintillation light havingentered the photoelectric conversion layer 31 uniformly enter theplurality of photoelectric conversion elements 32, the intensity of thescintillation light, namely, the current value accurately reflected withintensity of the radiation having entered the radiation detector 30 canbe detected in the photoelectric converter 35.

FIG. 3 is a diagram illustrating a circuit configuration of the signalprocessor. A circuit block configuration of the signal processor 22according to the present embodiment will be described with reference toFIG. 3.

As illustrated in FIG. 3, the signal processor 22 includes adifferential converter 220, an integrator 221 (integration unit), asampler and holder 222, an AD converter 223, a differentiator 224, azero cross detector 225, a time detector 226 (which may be referred toas an event interval detector), a counter circuit 227 (which may besimply referred to as a counter), a histogram creation circuit 228(which may be simply referred to as a creator), and a control circuit229 (which may be referred to as a pile-up detector).

The differential converter 220 receives current I detected by thephotoelectric converter 35 and reflected with the intensity of theradiation, converts the same to a differential signal in which noiseresistance performance is improved, and transmits the differentialsignal of the current to the integrator 221.

The integrator 221 executes integration processing for the currentreceived from the differential converter 220 and converted to thedifferential signal to calculate charge Q, and transmits the calculatedcharge Q to the sampler and holder 222. Specifically, the integrator 221includes a capacitor inside thereof, and when the current flows into thecapacitor, the current is accumulated as the charge Q. Therefore, avalue output by the integrator 221 is to be a value of voltage at bothends of the capacitor and the value is proportional to the chargeaccumulated in the capacitor. However, since the charge ismathematically obtained by integrating the current, the followingdescription will be provided assuming that the value output by theintegrator 221 is the value of the charge. The integrator 221 startsintegration processing at the timing of receiving the current from thedifferential converter 220 and at the timing of receiving alater-described pile-up detection signal from the control circuit 229.Furthermore, for example, the integrator 221 may perform discharge whenthe charge Q to be transmitted to the sampler and holder 222 andaccumulated in the capacitor is sampled and held by the sampler andholder 222 or when the charge Q sampled and held by the sampler andholder 222 is applied with AD conversion by the AD converter 223described later. Furthermore, the integrator 221 may receive theabove-described pile-up detection signal during the integrationprocessing performed by accumulating the charge Q in the capacitor orduring discharge of the accumulated charge Q. In this case, for example,the integrator 221 includes a plurality of capacitors, and in the caseof receiving the pile-up detection signal during the integrationprocessing or during discharge in a certain capacitor, the integratormay perform the integration processing in a different capacitor forcurrent based on a later-described pile-up waveform.

The sampler and holder 222 samples and holds the charge Q received fromthe integrator 221 as charge Q1 at the time of receiving a hold signalfrom the control circuit 229 as described later. Then, the sampler andholder 222 transmits the sampled and held charge Q1 to the AD converter223. Meanwhile, timing when the control circuit 229 transmits the holdsignal to the sampler and holder 222 will be described later.Furthermore, the sampler and holder 222 may cancel sampling and holdingwhen AD conversion is applied to the sampled and held charge Q1 by theAD converter 223 as described later or when the hold signal receivedfrom the control circuit 229 becomes Low (OFF). Additionally, thesampler and holder 222 can perform sampling and holding, and cancancellation the same for each of the plurality of capacitors includedin the integrator 221.

The AD converter 223 applies AD conversion to the charge Q1 receivedfrom the sampler and holder 222 in accordance with a predetermined clockfrequency, and transmits a digital signal of the charge Q1 to thecounter circuit 227. In other words, in the case where later-describedpile-up does not occur, the clock frequency by the AD converter 223 isto be an upper limit value of a count rate up to which the number oftimes when the scintillation light enters the photoelectric converter35, namely, the number of events can be counted. Meanwhile, the clockfrequency may be based on a clock generated inside by the AD converter223, and may also be based on a control signal transmitted from thecontrol circuit 229. Furthermore, for example, a plurality of ADconverters 223 having different clock frequencies may also be providedcorresponding to the plurality of capacitors included in the sampler andholder 222. This can increase the upper limit value of the count rate upto which the above-described number of events can be counted.

The differentiator 224 receives the current I detected by thephotoelectric converter 35 and reflected with the intensity of theradiation, calculates a differentiation value dI/dt by applyingdifferential processing to the current I, and transmits a calculateddifferential value to the zero cross detector 225.

The zero cross detector 225 detects a zero cross of a differentiatedwaveform from a waveform of the differential value (differentiatedwaveform) received from the differentiator 224, and transmits zero crossinformation to the time detector 226 and the control circuit 229 at thetime of detecting the zero cross. The zero cross information includesinformation of a zero cross when the differential value in thedifferentiated waveform becomes a positive value from a negative value(hereinafter referred to as rising zero cross) or a zero cross when thedifferential value becomes a negative value from a positive value(hereinafter referred to as falling zero cross). Also, the time when thezero cross is detected is the timing when inclination of a waveform ofthe current I becomes “zero”. Among them, the time when the falling zerocross is detected corresponds to the timing when the waveform of thecurrent I reaches a peak, and the time when the rising zero cross isdetected corresponds to the timing when later-described pile-up occurs.

The time detector 226 detects, based on the zero cross information fromthe zero cross detector 225, a period from a peak to a peak of thewaveform of the current I (later-described normal waveform or pile-upwaveform illustrated in FIG. 7), namely, an event interval. For example,in the case of receiving falling zero cross information after receivingfalling zero cross information, the time detector 226 detects the eventinterval from a peak of the normal waveform to a peak of a next normalwaveform or an event interval from a peak of a pile-up waveform that hasbeen piled up on the normal waveform to a peak of a next normalwaveform. Furthermore, in the case of receiving falling zero crossinformation, rising zero cross information, and then falling zero crossinformation in this order, the time detector 226 determines and detectsa period from timing of receiving the first falling zero crossinformation to timing of receiving the final falling zero crossinformation as an event interval as follows. More specifically, theperiod corresponds to an event interval from a peak of the normalwaveform to a peak of the pile-up waveform that has been piled up on thenormal waveform or an event interval from a peak of a pile-up waveformthat has been piled up on the normal waveform (or pile-up waveform) to apeak of a pile-up waveform that is additionally piled up on the pile-upwaveform. Then, the time detector 226 transmits the detected eventinterval to the counter circuit 227.

The counter circuit 227 receives information of the charge Q1 appliedwith AD conversion by the AD converter 223 and information of the eventinterval detected by the time detector 226. Meanwhile, in FIG. 7described later, charge of the normal waveform is defined as Q1 andcharge of the pile-up waveform is defined as Q2, but the charge Q1 hereincludes both. The counter circuit 227 counts the number of events(event number) in accordance with respective values of the receivedcharge Q1 and the event interval. Here, as a method of counting thenumber of events, as for the event corresponding to the normal waveform,the number of events may be counted in the case of receiving theinformation of the charge Q1. Furthermore, as for the eventcorresponding to the pile-up waveform, counting may be executed in thecase of receiving the information of the charge Q1 (in FIG. 7, indicatedas charge Q2) or in the case of receiving, from the control circuit 229,a control signal indicating transmission of a pile-up detection signalwhen the pile-up detection signal is transmitted from the controlcircuit 229 to the integrator 221. Then, the counter circuit 227transmits, to the histogram creation circuit 228, the information of thenumber of events corresponding to the respective values of the charge Q1and the event interval.

The histogram creation circuit 228 creates a histogram for the charge Q1and the number of events and a histogram for the event interval and thenumber of events based on information of the number of eventscorresponding to the respective values of the charge Q1 and the eventinterval received from the counter circuit 227. The histogram creationcircuit 228 transmits the created histogram information to an externalapparatus. Meanwhile, the histogram creation circuit 228 is not limitedto creating both of the histogram for the charge Q1 and the number ofevents and the histogram for the event interval and the number ofevents, and may also create one of these histograms.

The control circuit 229 controls entire operation of the signalprocessor 22. Specifically, the control circuit 229 receives the zerocross information from the zero cross detector 225, and in the casewhere the zero cross information relates to a rising zero cross, thecontrol circuit 229 detects occurrence of pile-up and transmits apile-up detection signal to the integrator 221. Furthermore, in the casewhere the zero cross information is received from the zero crossdetector 225 and the zero cross information relates to a falling zerocross, the control circuit 229 detects a fact that a waveform of thecurrent I (normal waveform and pile-up waveform) has reached a peak, andtransmits a hold signal to the sampler and holder 222. Meanwhile, it hasbeen described that the control circuit 229 transmits the hold signal atthe time of receiving the zero cross information including theinformation of the falling zero cross, but not limited thereto, thecontrol circuit 229 may also transmit the hold signal after apredetermined period has passed from receipt of the zero crossinformation, for example. Additionally, the control circuit 229transmits a control signal adapted to specify respective operation ofthe AD converter 223, counter circuit 227, and histogram creationcircuit 228. For example, in the case of detecting occurrence of pile-upas described above, the control circuit 229 may also transmit a controlsignal indicating such detection to the counter circuit 227.

Meanwhile, FIG. 3 is a diagram illustrating the exemplary configurationof the circuit block of the signal processor 22, and not limitedthereto, as far as functions of the above-described respectiveprocessors and respective circuits are provided, any configuration maybe applied.

FIG. 4 is an explanatory diagram for Poisson distribution related to anarrival probability of radiation. The arrival probability of theradiation emitted from the radiation tube 11 and arriving at thephotoelectric converter 35 of the radiation detecting unit 20, namely, aprobability of event occurrence will be described with reference to FIG.4.

The arrival probability of the radiation emitted from the radiation tube11 and arriving at the photoelectric converter 35 (probability of eventoccurrence) is represented by, for example, probability distribution 300in FIG. 4 basically in accordance with Poisson distribution as describedabove. The probability distribution 300 illustrated in FIG. 4 is a graphindicating an arrival probability of next radiation after a certainevent interval upon arrival of the radiation. As described above, sincethe radiation arrives at the photoelectric converter 35 at a higharrival rate of about 10⁸ [cps], the longer the event interval is, thelower the probability is as illustrated in the probability distribution300 of FIG. 4. Furthermore, since the probability distribution 300 isdistribution of the probability, an entire graph of the probabilitydistribution 300 is integrated, a value becomes “1” (area of the graphof the probability distribution 300).

Frequency distribution 301 illustrated in FIG. 4 is a graph indicating aprobability of next radiation arriving within a certain event intervalafter arrival of the radiation. For example, in the frequencydistribution 301, in the case where the event interval is 10 [ns], theprobability of the next radiation arriving within this 10 [ns] is “0.6”.Additionally, in the probability distribution 300, since the area of aportion between 0 and 10 [ns] indicates the probability of the nextradiation arriving within 10 [ns] after arrival of the radiation, thevalue becomes “0.6”.

FIG. 5 is an explanatory diagram for a case where an interval between anormal waveform and a pile-up waveform is wide. FIG. 6 is an explanatorydiagram for a case where the interval between the normal waveform andthe pile-up waveform is narrow. With reference to FIGS. 5 and 6,pile-up, integration operation by the integrator 221, and sampling andholding operation by the sampler and holder 222 will be described.

First, the pile-up and the integration operation by the integrator 221will be described with reference to FIG. 5. When radiation from theradiation tube 11 enters the photoelectric converter 35 and a photonenters the APD of the photoelectric conversion element 32, currentindicated by a normal waveform 400 of FIG. 5(a) flows. Since thephotoelectric conversion element 32 has the quench resistor connected tothe APD in series, the current indicated by the normal waveform 400 doesnot immediately becomes “zero” and a tail portion remains. As describedabove, since the arrival rate of the radiation is high, next radiationmay arrive before the tail portion is sufficiently lowered. In thiscase, the current generated by the radiation arriving next is added tocurrent generated by first radiation, and a signal higher than a signalof intrinsic current (normal waveform 401 of FIG. 5(a)) is generated.This is called pile-up. In FIG. 5(a), the normal waveform 401 based onthe next radiation is piled up on the normal waveform 400 that is thewaveform of the current based on the first radiation, and a waveformappearing as a signal having a height higher than a height of a signalof the intrinsic normal waveform 401 is indicated by a pile-up waveform402.

FIG. 5(b) is a diagram of a waveform of a differential value dI/dtobtained by differentiating the current I illustrated in FIG. 5(a). InFIG. 5(b), a differentiated waveform 410 obtained by differentiating thenormal waveform 400, a differentiated waveform 411 obtained bydifferentiating the normal waveform 401, and a pile-up differentiatedwaveform 412 obtained by differentiating the pile-up waveform 402 areillustrated. In these differentiated waveforms, when the differentialvalue dI/dt becomes “zero” at the time of becoming from a positive valueto a negative value, and when the differential value becomes “zero” atthe time of becoming from a negative value to a positive value, thedifferential value becomes a zero cross at which inclination in thecurrent waveform of FIG. 5(a) becomes “zero”. Specifically, asillustrated in FIG. 5, timing when a falling zero cross is detected inthe case where the differential value dI/dt is changed from the positivevalue to the negative value corresponds to the timing when a currentwaveform reaches a peak. Furthermore, timing when a rising zero cross isdetected in the case where the differential value dI/dt is changed fromthe negative value to the positive value corresponds to the timing whenpile-up occurs. As described above, when the falling zero cross or therising zero cross is detected, the zero cross detector 225 transmits thezero cross information including such information to the time detector226 and the control circuit 229.

FIG. 5(c) is a diagram illustrating an integration value obtained byintegrating the current I illustrated in FIG. 5(a), namely, a waveformof the charge Q. In FIG. 5(c), a charge waveform 420 obtained byintegrating the normal waveform 400, a charge waveform 421 obtained byintegrating the normal waveform 401, and a pile-up charge waveform 422obtained by integrating the pile-up waveform 402 are illustrated. Thecharge indicated by these charge waveforms is calculated by integrationoperation by the integrator 221. However, among the charge waveformsillustrated in FIG. 5(c), the charge waveform 421 indicates the waveformof the charge in the case where the normal waveform 401 is integrated,and since the normal waveform 401 is piled up on the normal waveform400, the normal waveform 401 is not actually calculated by beingintegrated by the integrator 221.

Next, sampling and holding operation by the sampler and holder 222 willbe described with reference to FIG. 6. FIG. 6(a) is a diagramillustrating an exemplary case where an interval between a peak of thenormal waveform 400 and a peak of the normal waveform 401 (eventinterval) is narrower than the event interval illustrated in FIG. 5(a).FIG. 6(b) is a diagram illustrating a waveform of a differential valuedI/dt obtained by differentiating the current I illustrated in FIG.6(a). FIG. 6(c) is a diagram illustrating an integration value obtainedby integrating the current I illustrated in FIG. 6(a), namely, awaveform of the charge Q.

First, the charge Q that is a result value of integrating currentgenerated by an event is calculated by using the integrator 221 in orderto acquire intensity of radiation having generated the event. However;the charge Q is hardly calculated by singularly integrating an entirecurrent value of the normal waveform 400 because the normal waveform 401is piled up on the normal waveform 400 as illustrated in FIGS. 5 and 6.Accordingly, the signal processor 22 according to the present embodimentexecutes operation to read an integration value (charge) of a waveformat a portion of the normal waveform 400 not influenced by the pile-upwaveform 402, and deems a magnitude of this integration value as a valuereflected with intensity of the radiation. Specifically, the controlcircuit 229 transmits a hold signal to the sampler and holder 222 at thetime of receiving zero cross information related to a falling zero crossfrom the zero cross detector 225 or after a predetermined period haspassed from receipt of the zero cross information related to the fallingzero cross. Consequently, as illustrated in FIG. 6(c), the charge Qintegrated by the integrator 221 at a portion of an integration region500 of the normal waveform 400 not influenced by the pile-up waveform402 is sampled and held in the charge Q1 by the sampler and holder 222.Then, the sampled and held charge Q1 is applied with AD conversion bythe AD converter 223 in accordance with a predetermined clock frequency,and transmitted to the counter circuit 227 as a digital signal. Thecharge Q1 is the charge of the waveform at the portion of the normalwaveform 400 not influenced by the pile-up waveform 402, and is regardedas the value reflected with intensity of the radiation.

Furthermore, as illustrated in FIG. 6(a), when the interval between thepeak of the normal waveform 400 and the peak of the normal waveform 401becomes narrow, a wave height at the peak of the pile-up waveform 402becomes higher than a wave height of the pile-up waveform 402illustrated in FIG. 5(a). In other words, when pile-up occurs, a waveheight value of the peak of the pile-up waveform 402 depends on theinterval between the normal waveform 400 and the normal waveform 401(event interval). Therefore, it is difficult to directly calculate thecharge Q obtained singularly by the normal waveform 401. Therefore,utilizing the fact that the wave height value of the peak of the pile-upwaveform 402 depends on the event interval between the normal waveform400 and the normal waveform 401, the counter circuit 227 estimatescharge of the singular normal waveform 401, corresponding to the chargeQ1 of the normal waveform 400 as described below. In other words, whenthe counter circuit 227 receives the control signal indicating thepile-up from the control circuit 229, the counter circuit 227 estimatesthe charge of the normal waveform 401 corresponding to the charge Q1based on the charge Q1 obtained from the normal waveform 400 and theevent interval between the normal waveform 400 and the pile-up waveform402 (can also be referred to as the event interval between the normalwaveform 400 and the normal waveform 401). The normal waveform 401 is anintrinsic current waveform in the case of assuming that the pile-upwaveform 402 is not piled up.

FIG. 7 is a timing chart from entrance of radiation to application of ADconversion. Operation from when the radiation is made to enter thephotoelectric converter 35 to when the sampled and held charge isapplied with AD conversion will be described with reference to FIG. 7.In FIG. 7, first current indicated by a normal waveform 600 is generatedby the radiation made to enter the photoelectric converter 35. Next,current indicated by a normal waveform 601 is generated by the radiationmade to enter the photoelectric converter 35 subsequently. The radiationfurther enters the photoelectric converter 35 at timing in a tableportion of this normal waveform 601, and current indicated by a pile-upwaveform 601 a piled up on the normal waveform 601 is generated.Furthermore, subsequently, current indicated by a normal waveform 602 isgenerated by the radiation made to enter the photoelectric converter 35.Meanwhile, wave height values of peaks of the normal waveforms 600 to602 are illustrated to be the same heights in FIG. 7, but actually, thewave height values (intensity of radiation) are varied by the number ofphotoelectric conversion elements 32, where photons of the radiationenter, out of the plurality of photoelectric conversion element 32 ofthe photoelectric converter 35.

First, a description will be provided for a processing flow relative tothe normal waveform 600 that is the waveform of the current generated bythe radiation made to enter the photoelectric converter 35 and flowingin the photoelectric conversion element 32.

The differential converter 220 receives the current I detected by thephotoelectric converter 35, reflected with intensity of the radiation,and indicated by the normal waveform 600, converts the current I to adifferential signal, and transmits the differential signal to theintegrator 221. The integrator 221 executes integration processing forthe current (differential signal) received from the differentialconverter 220, calculates the charge Q indicated by a charge waveform620, and transmits the calculated charge Q to the sampler and holder222. The differentiator 224 receives the current I detected by thephotoelectric converter 35, reflected with the intensity of theradiation, and indicated by the normal waveform 600, and calculates adifferentiation value dI/dt by applying differential processing to thecurrent I, and then transmits a calculated differential value to thezero cross detector 225. The zero cross detector 225 detects a fallingzero cross from the differentiated waveform 610 that is the waveform ofthe differential value received from the differentiator 224, andtransmits zero cross information to the time detector 226 and thecontrol circuit 229 at the time of detecting the falling zero cross.

The control circuit 229 detects a fact that the normal waveform 600 ofthe current I has reached a peak by receiving the zero cross informationrelated to the falling zero cross from the zero cross detector 225, andtransmits a hold signal SH1 to the sampler and holder 222. Meanwhile, ithas been described that the control circuit 229 transmits the holdsignal SH1 at the time of receiving the zero cross information relatedto the falling zero cross, but not limited thereto, the control circuit229 may also transmit the hold signal SH1 after a predetermined periodhas passed from receipt of the zero cross information, for example. Thesampler and holder 222 samples and holds the charge Q received from theintegrator 221 as charge Q1 at the time of receiving the hold signal SH1from the control circuit 229. Then, the sampler and holder 222 transmitsthe sampled and held charge Q1 to the AD converter 223.

The AD converter 223 applies AD conversion to the charge Q1 receivedfrom the sampler and holder 222 in accordance with a clock signal ADC ofa predetermined clock frequency, and transmits a digital signal of thecharge Q1 to the counter circuit 227. Furthermore, the sampler andholder 222 may cancel sampling and holding when AD conversion is appliedto the sampled and held charge Q1 by the AD converter 223 or when thehold signal SH1 received from the control circuit 229 becomes Low (OFF).Furthermore, the integrator 221 performs discharge when the accumulatedcharge Q is sampled and held by the sampler and holder 222 or when ADconversion is applied by the AD converter 223 to the charge Q1 sampledand held by the sampler and holder 222.

The time detector 226 detects an event interval from a peak of thenormal waveform 600 to a peak of the normal waveform 601 of the currentI based on the zero cross information related to the falling zero crossand later-described falling zero cross information relative to thenormal waveform 601 received from the zero cross detector 225. Then, thetime detector 226 transmits the detected event interval to the countercircuit 227. In the flow described above, the signal processor 22executes the processing for the normal waveform 600 that is the waveformof the current generated by the radiation first made to enter thephotoelectric converter 35 and flowing in the photoelectric conversionelement 32.

Next, a description will be provided for a processing flow for thenormal waveform 601 that is the waveform of the current generated by theradiation made to enter the photoelectric converter 35 after occurrenceof an event related to the normal waveform 600 and flowing in thephotoelectric conversion element 32, and the pile-up waveform 601 a thatis the waveform of the current generated by the radiation subsequentlymade to enter, flowing in the photoelectric conversion element 32, andpiled up on the normal waveform 601.

The differential converter 220 receives the current I detected by thephotoelectric converter 35, reflected with intensity of the radiation,and indicated by the normal waveform 601, converts the current I to adifferential signal, and transmits the differential signal to theintegrator 221. The integrator 221 executes integration processing forthe current (differential signal) received from the differentialconverter 220, calculates charge Q indicated by the charge waveform 621,and then transmits the calculated charge Q to the sampler and holder222. The differentiator 224 receives the current I detected by thephotoelectric converter 35, reflected with the intensity of theradiation, and indicated by the normal waveform 601, and calculates adifferentiation value dI/dt by applying differential processing to thecurrent I, and then transmits the calculated differential value to thezero cross detector 225. The zero cross detector 225 detects a fallingzero cross from the differentiated waveform 611 that is the waveform ofthe differential value received from the differentiator 224, andtransmits zero cross information to the time detector 226 and thecontrol circuit 229 at the time of detecting the falling zero cross.

The control circuit 229 detects a fact that the normal waveform 601 ofthe current I has reached a peak by receiving the zero cross informationrelated to the falling zero cross from the zero cross detector 225, andtransmits a hold signal SH1 to the sampler and holder 222. Meanwhile, ithas been described that the control circuit 229 transmits the holdsignal SH1 at the time of receiving the zero cross information relatedto the falling zero cross, but not limited thereto, the control circuit229 may also transmit the hold signal SH1 after a predetermined periodhas passed from receipt of the zero cross information, for example. Thesampler and holder 222 samples and holds the charge Q received from theintegrator 221 as charge Q1 at the time of receiving the hold signal SH1from the control circuit 229. Then, the sampler and holder 222 transmitsthe sampled and held charge Q1 to the AD converter 223.

The time detector 226 detects an event interval from the peak of thenormal waveform 600 to the peak of the normal waveform 601 of thecurrent I based on the zero cross information related to the fallingzero cross of the above described normal waveform 600 and the fallingzero cross information of the normal waveform 601 received from the zerocross detector 225. Then, the time detector 226 transmits the detectedevent interval to the counter circuit 227.

The zero cross detector 225 further detects a rising zero cross from thedifferentiated waveform 611 that is the waveform of the differentialvalue received from the differentiator 224, and transmits zero crossinformation to the time detector 226 and the control circuit 229 at thetime of detecting the rising zero cross. The control circuit 229 detectsoccurrence of pile-up on the normal waveform 601 of the current I byreceiving the zero cross information related to the rising zero crossfrom the zero cross detector 225, and transmits a pile-up detectionsignal to the integrator 221. The integrator 221 starts integrationprocessing for the current received from the differential converter 220and indicated by the pile-up waveform 601 a at the timing of receivingthe pile-up detection signal from the control circuit 229, calculatesthe charge Q indicated by a charge waveform 621 a, and transmits thecalculated charge Q to the sampler and holder 222. Here, since theintegration processing has been executed by a specific capacitor for thecurrent indicated by the normal waveform 601 at the time of starting theintegration processing for the current indicated by the pile-up waveform601 a, the integrator 221 starts the integration processing by acapacitor different from the capacitor.

The zero cross detector 225 further detects a falling zero cross fromthe differentiated waveform 611 that is the waveform of the differentialvalue received from the differentiator 224, and transmits zero crossinformation to the time detector 226 and the control circuit 229 at thetime of detecting the falling zero cross. The control circuit 229detects a fact that the pile-up waveform 601 a of the current I hasreached the peak by receiving the zero cross information related to thefalling zero cross from the zero cross detector 225, and transmits ahold signal SH2 to the sampler and holder 222. Meanwhile, it has beendescribed that the control circuit 229 transmits the hold signal SH2 atthe time of receiving the zero cross information related to the fallingzero cross, but not limited thereto, the control circuit 229 may alsotransmit the hold signal SH2 after a predetermined period has passedfrom receipt of the zero cross information, for example. The sampler andholder 222 samples and holds the charge Q received from the integrator221 related to the pile-up waveform 601 a as charge Q2 at the time ofreceiving the hold signal SH2 from the control circuit 229. Then, thesampler and holder 222 transmits the sampled and held charge Q2 to theAD converter 223.

The AD converter 223 applies AD conversion to the charge Q1 receivedfrom the sampler and holder 222 in accordance with a clock signal ADC ofa predetermined clock frequency, applies AD conversion to the charge Q2in accordance with a next clock signal ADC, and transmits digitalsignals of the charge Q1, Q2 to the counter circuit 227. Furthermore,the sampler and holder 222 may cancel sampling and holding when ADconversion is applied to the sampled and held charge Q1, Q2 by the ADconverter 223 or when the hold signals SH1, SH2 received from thecontrol circuit 229 become Low (OFF). Furthermore, the integrator 221perform discharge when the accumulated charge Q is sampled and held bythe sampler and holder 222 or when AD conversion is applied by the ADconverter 223 to the charge Q1, Q2 sampled and held by the sampler andholder 222. Meanwhile, AD conversion for the charge Q1 obtained byintegrating the current of the normal waveform 601 with the integrator221 and being sampled and held by the sampler and holder 222. and ADconversion for the charge Q2 obtained by integrating the current of thepile-up waveform 601 a by the integrator 221 and being sampled and heldby the sampler and holder 222 may be applied by a plurality of ADconverters 223 having different clock frequencies. In other words, ADconversion may be consecutively applied to the charge Q1 and charge Q2by applying AD conversion to the charge Q1 and making the hold signalSH2 High (ON) until timing of next AD conversion having a differentclock frequency.

The time detector 226 detects an event interval from the peak of thenormal waveform 601 to a peak of the pile-up waveform 601 a of thecurrent I based on the zero cross information related to the fallingzero cross of the above-described normal waveform 601 and the fallingzero cross information of the pile-up waveform 601 a received from thezero cross detector 225. Then, the time detector 226 transmits thedetected event interval to the counter circuit 227. In theabove-described flow, the signal processor 22 executes the processingfor the normal waveform 601 and the pile-up waveform 601 a piled up onthe normal waveform 601.

Meanwhile, even in the case where pile-up further occurs on the pile-upwaveform 601 a, the operation is the same as described above.

FIG. 8 is a diagram illustrating an exemplary histogram for the chargeand the number of events. FIG. 9 is a diagram illustrating an exemplaryhistogram for the event interval and the number of events. Eventcounting operation by the counter circuit 227 and histogram creatingoperation by the histogram creation circuit 228 will be described withreference to FIGS. 8 and 9.

The counter circuit 227 receives information of the charge (charge Q1,Q2 in FIG. 7) applied with AD conversion by the AD converter 223 andinformation of the event interval detected by the time detector 226. Thecounter circuit 227 counts the number of events (event number) inaccordance with respective values of the received charge Q1, Q2 andevent interval. Here, as a method of counting the number of events, asfor the event corresponding to the normal waveform, the number of eventsmay be counted in the case of receiving the information of the chargeQ1. Furthermore, as for the event corresponding to the pile-up waveform,counting may be executed in the case of receiving the information of thecharge Q2 or in the case of receiving, from the control circuit 229, acontrol signal indicating transmission of a pile-up detection signalwhen the pile-up detection signal is transmitted from the controlcircuit 229 to the integrator 221.

Furthermore, since a wave height value of the peak of the pile-upwaveform 601 a is the value piled up on the normal waveform 601, thewave height is not the value not reflected with intensity of intrinsicradiation, different from the wave height value of the peak of theintrinsic normal waveform (waveform of the current I indicated by adotted line in FIG. 7). Therefore, utilizing a fact that the wave heightvalue of, for example, the peak of the pile-up waveform 601 a in FIG. 7depends on an event interval between the normal waveform 601 and theintrinsic normal waveform related to the pile-up waveform 601 a(waveform of current I indicated by the dotted line in FIG. 7), thecounter circuit 227 estimates charge obtained singularly by the waveformof the current I indicated by the dotted line in FIG. 7 andcorresponding to the charge Q1 of the normal waveform 601. In otherwords, when the counter circuit 227 receives the control signalindicating pile-up from the control circuit 229, the counter circuit 227estimates the charge of the intrinsic normal waveform corresponding tothe charge Q1 based on the charge Q1 obtained from the normal waveform601 and the event interval between the normal waveform 601 and thepile-up waveform 601 a. The intrinsic normal waveform is the waveform inthe case of assuming that the pile-up waveform 601 a is not piled up.Then, the counter circuit 227 transmits, to the histogram creationcircuit 228, the information of the number of events corresponding tothe respective values of the charge and the event interval.

The histogram creation circuit 228 creates a histogram for the chargeand the number of events and a histogram for the event interval and thenumber of events based on information of the number of eventscorresponding to the respective values of the charge and the eventinterval received from the counter circuit 227. By acquiring suchhistograms, it is possible to specify a substance constituting the testobject 12 illustrated in FIG. 1 or recognize a composition and the likeof a tissue. The exemplary histogram for the charge and the number ofevents is illustrated in FIG. 8, and the exemplary histogram for theevent interval and the number of events is illustrated in FIG. 9. Thehistogram for the event interval and the number of events as illustratedin FIG. 9 shows the number of events (vertical axis) for respectiveevent intervals (horizontal axis), and indicates the number of events atwhich the radiation arrives at the event interval, and further indicatesprobability distribution for the respective event intervals inaccordance with increase of the number of events (number of samples).Therefore, the probability distribution should be the distributionhaving a shape similar to the probability distribution 300 in accordancewith the above-described Poisson distribution illustrated in FIG. 4.Therefore, in the histogram creation circuit 228, an average value, avariance value, a mode value, and the like may be exemplified ascharacteristic values of the Poisson distribution (probabilitydistribution 300) in which correction can be made on the createdhistogram for the event interval and the number of events based on thecharacteristic values of the Poisson distribution (probabilitydistribution 300) illustrated in FIG. 4. Thus, it can be expected that ahighly accurate histogram is obtained by correcting the histogram forthe event interval and the number of events created by the histogramcreation circuit 228 based on the characteristic values of the Poissondistribution.

As described above, occurrence of the event is counted by counting thenumber of charge of the normal waveform obtained by applying ADconversion in accordance with the clock signal of the clock frequency ofthe AD converter 223 and further detecting a fact that pile-up isdetected. Consequently, the event of the radiation that arrives with thearrival rate higher than the clock frequency of the AD converter 223 canbe detected with high accuracy. Furthermore, this enables creation ofthe histogram in which errors are reduced and accuracy is improved.

Additionally, as detection of the charge of the normal waveform, thecontrol circuit 229 makes the sampler and holder 222 sample and hold thecharge received from the integrator 221 at the time of receiving thezero cross information related to falling zero cross from the zero crossdetector 225 or after the predetermined period has passed from receiptof the zero cross information related to the falling zero cross.Consequently, the value of the charge of the normal waveform havingreduced influence from the pile-up waveform can be obtained as the valueof the normal waveform reflected with intensity of the radiation thatarrives at the photoelectric converter 35. Therefore, the histogram forthe charge and the number of events, in which errors are reduced andaccuracy is improved, can be created.

Furthermore, as an equivalent of the charge calculated for the normalwaveform as described above, the charge of the pile-up waveform isestimated based on the charge calculated for the normal waveform and theevent interval between the event related to the normal waveform and theevent related to the pile-up waveform. Consequently, the chargeequivalent to the charge calculated for the normal waveform can beaccurately obtained as the value related to the pile-up waveform andreflected with the intensity of the radiation having arrived at thephotoelectric converter 35. Accordingly, the histogram for the chargeand the number of events, in which errors are reduced and accuracy isimproved, can be created.

Furthermore, the histogram creation circuit 228 can make correction onthe created histogram for the event interval and the number of eventsbased on the characteristic values of the Poisson distribution.Consequently, the histogram for the event interval and the number ofevents, in which errors are reduced and accuracy is improved, can becreated.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A signal processor comprising: an integrator configured to calculate charge by integrating current from a photoelectric converter that converts to the current based on an incident radiation; a differentiator configured to calculate a differential value by differentiating the current; a zero cross detector configured to detect a zero cross of the differential value; a pile-up detector configured to detect pile-up relative to the current based on the zero cross; an event interval detector configured to detect, based on the zero cross and the pile-up, an event interval that is a time period between events in which the radiation enters the photoelectric converter; a counter configured to count, based on the charge and the pile-up, number of events according to the charge, and number of events according to the event interval; and a creator configured to create a histogram for the number of events relative to the charge, and a histogram of the number of events relative to the event interval.
 2. The signal processor according to claim 1, further comprising a holder configured to hold an integration value of the current from when the current for a normal waveform, which is not a pile-up waveform obtained by occurrence of the pile-up on the current, is started to be integrated by the integrator to a time point within a predetermined time period that has passed from detection of the zero cross of falling detected by the zero cross detector, wherein the counter is configured to count the number of events based on the charge that is the integration value held by the holder, and the pile-up.
 3. The signal processor according to claim 1, wherein the counter estimates the charge for a current waveform corresponding to an origin of the pile-up waveform for the current based on the charge calculated by the integrator for the normal waveform on which a pile-up waveform is piled up by occurrence of the pile-up on the current, and based on the event interval between the event relative to the normal waveform and the event relative to the pile-up waveform.
 4. The signal processor according to claim 1, wherein the creator corrects the histogram for the number of events relative to the event interval based on a characteristic value of Poisson distribution.
 5. A signal processor comprising: an integrator configured to calculate charge by integrating current from a photoelectric converter that converts to the current based on an incident radiation; a differentiator configured to calculate a differential value by differentiating the current; a zero cross detector configured to detect a zero cross of the differential value; and a controller configured to detect pile-up relative to the current based on the zero cross, wherein the integrator includes a plurality of capacitors, and if the controller detects pile-up when the current is being integrated using a capacitor of the capacitors, the integrator is configured to integrate using another capacitor of the capacitors the current corresponding to a pile-up waveform obtained by occurrence of the pile-up.
 6. The signal processor according to claim 5, further comprising a holder configured to hold an integration value of the current from when the current for a normal waveform, which is not the pile-up waveform obtained by occurrence of the pile-up on the current, is started to be integrated by the integrator to a time point within a predetermined time period that has passed from detection of a first zero cross of falling detected by the zero cross detector.
 7. The signal processor according to claim 6, wherein the holder is configured to hold an integration value of the current corresponding to the pile-up waveform from when the current corresponding to the pile-up waveform is started to be integrated by the integrator to a time point within a predetermined time period that has passed from detection of a second zero cross of falling detected by the zero cross detector next to the first zero cross of falling.
 8. A radiation detection device comprising: a signal processor according to claim 1; a scintillator configured to convert the radiation to scintillation light having a wavelength longer than a wavelength of the radiation; the photoelectric converter configured to convert the scintillation light to the current. 